“On Call” means you are waiting for someone to call you. When they call you then you are available to come and help them.

Hello Baboon, "On call" is a term used in certain industries, mainly the medical field, which means you are ready anytime your help is needed. Hope this helps.

I agree that more context would be useful to help us be certain about the meaning, but I think it's likely that 'call' is: Either a verb to indicate one person giving an instruction to prepare to do something when told. For example a leader in the army might say "Prepare to fire on my call." (In other words "Don't fire until I say") Or a noun that describes a telephone conversation (or maybe a video call in a modern context) For example "Can you give me feedback on my call?" (In other words "Tell me about my performance during the conversation.")

It could mean different things. A sentence is always helpful.